1. Field
The present disclosure generally relates to accurate and specific control of water oxidation reduction potentials, and more particularly to systems, methods and apparatus for safe and effective water sanitation and treatment.
2. General Background
Various methods and apparatus have been utilized in order to treat/sanitize water. For example, the use of oxidants such as gaseous ozone for disinfection is well known. Typically, retention chambers are utilized into which ozone is introduced to water contained therein. Oxidation-reduction reactions then take place between the introduced ozone and contaminants in the water, where the oxidants are reduced and contaminants in the water are oxidized. Various oxidants are well known in the water treatment arts, such as bromine and chlorine, for example.
A common problem with such prior art systems is the reliance on less than accurate/controllable methods for monitoring and controlling the amount of residual oxidant (e.g. ozone) retained in water after introduction of the oxidant into water to be treated. Another troublesome aspect is the production of various radicals and side reactions that result in residual oxidizing species. Various methodologies have been employed to control oxidant-contaminant reactions. One prior art method utilizes multiple chambers to allow the introduction of an oxidant, ozone, to break down and oxidize contaminants, the remaining ozone then dissipating into oxygen. In such a system, a main component is time. That is, there is a passive reliance on the inherent breakdown of the oxidant introduced into the system. Additionally, when an oxidant is introduced at a consistent rate or amount into a process stream of water to be treated, fluctuations in the amount of contaminants in the water to be treated greatly affects the reaction dynamics between the introduced oxidant and the contaminant. Reductions in the amount of contaminants, or those compounds to be oxidized in a process stream, without an accurate and concordant reduction of introduced oxidant will lead to unacceptably high concentrations of residual oxidants in the process stream.
This typically leads to introduction of an oxidant at unacceptable levels into a water system or water source. This is particularly an issue when the destination of this treated water includes/supports various life forms that will be adversely affected by the introduction of treated water having unacceptably high concentrations of residual oxidants. Inaccurate prior art chemical methods for neutralizing oxidants introduced to sanitize water typically result in unwanted chemical reactions that can be detrimental, particularly when water in which such reactions are introduced into an aquatic ecosystem.
Other prior art methodologies include technology utilizing oxidation reduction potential monitoring for controlling oxidant feed. Typically these methods regulate oxidant feed based on how the oxidant is consumed, reacting with target substances/contaminants and unwanted organisms, within a system. As an example, a typical prior art method for treating water utilizes dissolved ozone as an oxidant and hydrogen peroxide to decompose remaining ozone concentrations left after the passage of a set amount of time. The addition of peroxide merely creates a less stable and more reactive oxidant that is less likely to persist. Allowing for the natural decay of the oxidant presents some major limitations to these technologies.
One limitation of the prior art is the fact that the rate of oxidant feed is limited by the demand and ability of the target system to remove it. This often prevents one from being able to dose an oxidant at high enough rates and/or concentrations to effect complete sterilization/sanitation. For example, Cryptosporidium is a significant health hazard for humans that can cause life threatening diarrhea. This pathogen is highly resistant to all but very high oxidant concentrations, concentrations that may not be obtainable utilizing prior art methods due to the inability of such methods to effectively neutralize the high concentration of the oxidant in a useful manner. In the case of lagoons, reefs, or any sensitive ecosystem into which such treated water is introduced, the release of even the smallest amounts of oxidant is potentially life threatening to flora and fauna residing therein.
Hydrogen peroxide is a weak acid that is partially dissociated in water, based on the pH, into its hydroperoxide ion.
An equilibrium equation is, H2O2+H2OHO2−+H3O+, pKa=11.6
The hydrogen peroxide molecule itself reacts very slowly with ozone, conversely the hydroperoxide ion reacts very quickly. The actual reaction profile is very complex with the formation of multiple types of free radicals including the production of hydroxide radicals. It is through the production of these radicals, from the combination of ozone and hydrogen peroxide, that provides for the techniques of prior art advanced oxidation processes for a variety of water remediation challenges. The mechanism of ozone decomposition, initiation and propagation reactions are proposed as follows (Ozone in Water Treatment: Application and Engineering, 1991):H2O2+H2OHO2−+H3O+O3+HO2−→OH+O2−+O2 O2−+H+HO2 O3+O2−→O3−+O2 HO3→OH+O2 
As can be seen from the equations above, the actual decomposition of ozone by hydrogen peroxide is fairly complex and includes the production of hydroxide and superoxide radicals. The products of these reactions will provide for further oxidation of oxidizable organics and/or inorganics.
As can be seen from the above equations, the use and addition of hydrogen peroxide into a process stream to control or neutralize dissolved residual ozone will decompose ozone molecules, but in the process create unwanted free radical residuals along with some remaining unreacted hydrogen peroxide that will contribute to elevated oxidation reduction potentials of water in a treated effluent stream. Such reaction remnants are highly undesirable and indeed may be detrimental to flora and fauna that reside in water into which such treated effluent streams may be introduced.
An amusement park aquarium system is an example where accurate control of a process stream of water is required and unwanted free radical residuals along with unreacted hydrogen peroxide, are not desired. Amusement park aquarium systems typically house substantially synthetic seawater. These systems can be quite large, holding and maintaining millions of gallons of seawater. These systems are typically closed in that no water is added or removed except through evaporation and slight operation losses. Seawater in aquatic displays typically supports various aquatic life forms. As such, the water contained therein receives significant waste products/contaminants from marine mammals and fish that reside therein, in addition to the various plant, color bodies and other contaminants typically found in such displays.
Excess waste products result in organic build up and color bodies that render waters in such displays uninhabitable. The buildup also limits visibility to patrons visiting the aquarium. For example, seawater in such aquariums take on a significant green/yellow cast that limits visibility and gives the aquarium an unhealthy and unnatural appearance.
Unfortunately, in prior art systems, the rate of oxidation is traditionally limited by the susceptibility of the resident aquatic species to tolerate byproducts produced by the oxidation reactions, such as hypobromous acid. Often the animals housed in these aquatic habitats are very sensitive to, and easily damaged by even slight residual amounts of ozone, chlorine, bromine, or other halogens. There are times when marginally acceptable water quality often takes precedent over increased oxidation treatments due to animal health concerns. Oxidation treatments would be effective at treating color and waste concerns, but the necessary dosing required would likely lead to harm the surrounding environment and animals residing therein.
There exist treatment systems for neutralizing oxidizing agents by delivering neutralizing and converting chemicals like sulfur dioxide, sodium thiosulphate and ascorbic acid. However, these systems are used almost exclusively with chlorine. Traditionally, the conversion chemicals are “dumped” wholesale into a process stream to completely erase any oxidative potential and there is no regulation of oxidative potential. These systems are typically used to de-chlorinate water before the water is released into surface water systems. The method of conversion is crude, largely uncontrolled, and potentially releases significant amounts of unreacted neutralizing chemicals into the environment. In a closed system such as a commercial aquarium, the unreacted neutralizing chemicals can cycle back through a process stream and deactivate oxidizing agents that are introduced and before they react with the target contaminants and harmful waste products. The uncontrolled release of the neutralizing chemicals also results in incomplete conversion of the oxidizing agents or harmful chemicals to safe compounds, which results in harm to the surrounding environment.
Water treatment systems utilized in other applications also experience similar problems. For example, watercrafts, such as cruise ships, must also disinfect discharge waters that are dumped into the ocean. Discharge waters are typically substantially made up of grey and/or black water that is generated onboard the watercraft. Grey water is typically used water from showers, sinks or basins, including used kitchen water. Black water is water contaminated with human waste, collected from shipboard toilets. Under various national and international standards, black water must be treated before being discharged from a vessel. During water treatment, undesirable by-products and unreacted oxidants discharged by these watercraft harm the environment and bodies of water in which these vehicles travel. In some cases, typically depending upon the types of water treatment system employed onboard and/or the location of the vessel, the watercraft are not allowed to discharge treated water into surrounding natural bodies of water. Often, watercraft must store the grey and/or black water generated onboard and transfer such water to a water treatment system located off board.